Imagers produced according to the CMOS (“Complementary Metal Oxide Semiconductor”) technology are currently the subject of an increasing number of applications due to their low cost price in comparison with CCD (Charge Coupled Device) imagers. Such CMOS imagers were initially used to produce low resolution image sensors of mediocre quality (for example web cameras). Today, after major investment in research and development, CMOS imagers can compete with CCD imagers.
FIG. 1 represents an example of a module for capturing images and/or video using a CMOS imager, intended for example to be mounted into a portable device such as a mobile telephone, a camera or a video camera. The module 1 comprises a frame 2, an optical set or lens-holder block 3, an objective based on lenses 4 mounted in the block 3 with a diaphragm 5, an infrared filter 6 and a base 7. A CMOS imager 11 having the shape of a semiconductor chip is disposed on the base 7 so as to receive the light passing through the lenses 4, the diaphragm 5 and the infrared filter 6.
The CMOS imager 11 comprises a plurality of photosites each forming a pixel (not visible in FIG. 1). Each pixel comprises a photodiode and a control and interconnection circuit of the photodiode. The pixels are arranged as an array and a mosaic of red, green and blue filters is distributed over the pixel array, generally according to the Bayer architecture (the cells of a line being alternately red and green, or alternately green and blue). Each pixel is thus covered with a determined, red, green or blue, primary color filter and provides a piece of luminance information about the primary color allocated to it, forming a piece of pixel information.
FIG. 2 is a schematic cross-section of the CMOS imager 11 in a region corresponding to three pixels PIX1, PIX2, PIX3. Going from bottom to top, layers 11a, 11b, 11c, 11d, 11e and microlenses L0 (L0-1, L0-2, L0-3) can be distinguished. The layer 11a is the semiconductor substrate in which the imager is implanted. This layer 11a thus represents the active part of the imager and comprises photodiodes and their associated control and interconnection circuits (not detailed). The layer 11b is formed by a dielectric material that entirely covers the substrate 11a. The layer 11c is a passivating layer deposited on the imager at the end of the CMOS manufacturing process. The layer 11d is formed by colored resins and comprises red, green or blue areas 11-1, 11-2, 11-3 forming the above-mentioned primary color filters, with one color filter per pixel. The layer 11e is an intermediate layer of resin forming a base for the microlenses L0 and providing good flatness. The microlenses L0 are arranged in a so-called “MLA” (“Microlens Array”) with one microlens per pixel.
The lenses 4 of the optical set are generally formed in molds by using a polymer resin that is removed from the molds after baking. Another known technique of manufacturing the lenses 4 involves printing polymer resin on a base, then the resin hot creeping to obtain a convex (rounded) face.
The microlenses L0 are also manufactured by using a polymer resin and the method for manufacturing the latter comprises for example depositing a layer of light-sensitive polymer resin (photoresist) onto an imager plate, soft baking, exposing the layer of resin to an ultraviolet light through an insolation mask and removing the insolated parts with an organic solvent to obtain an array of flat pads. The flat lenses are then subjected to a determined temperature to undergo a thermal creep that renders their upper face convex. They are then hardened by a final operation of annealing.
These manufacturing methods and particularly the method for manufacturing and mounting the objective lenses in the optical set, have the disadvantage of being complex, long to implement, and of involving many parts. As a result, these methods are costly.